1. Field of the Invention
The present invention pertains to formation of quasi-crystalline films by direct vapor deposition. The present invention overcomes difficulties of prior art techniques such as casting a quasi-crystalline alloy against a spinning wheel (melt spinning) and solid state transformation. More particularly, the present invention is a sputtering method which produces quasi-crystalline materials. It involves a direct vapor to solid transformation which achieves a much higher individual atomic quench rate than solidification from the melt as in melt spinning.
2. Quasi-crystals
Quasi-crystals are neither crystalline nor amorphous. They do not have long range crystalline order or belong to the classic fourteen point groups of crystalline materials. The quasi-crystalline materials have five-fold symmetry and near-crystalline order which is in contrast to the amorphous or glassy solids. The research over the last twenty years on rapidly quenched glassy metals (intermetallic glasses) has led to significant applications as corrosion-resistant and magnetic materials. The quasi-crystals were discovered less than two years ago. The advantages of these materials are analogous to the amorphous intermetallic glasses because they have effectively very fine grain size with excellent uniformity and therefore unique and superior potential as corrosion resistant alloys and as magnetic materials. The quasi-crystalline materials have advantages over the amorphous intermetallic glasses and alloys. They have higher stability at elevated temperatures and for longer time periods than amorphous material of the same composition. This higher uniformity than the crystalline makes quasi-crystalline films useful for laser discs, optical films, and corrosion or wear resistant films.
Quasi-crystals are discussed in a number of journal articles including the following which are incorporated herein by reference:
D. Schectman, I. Bleck, D. Gratias, and J. W. Cahn, Phys. Rev. Lett., Volume 53, No. 20, page 1951 (1984); P. J. Steinhardt, Quasi-Crystals, American Scientist, Volume 74, page 586 (1986); D. R. Nelson, Quasi-Crystals, Scientific American, Volume 255, No. 2, page 42 (1986).
To understand the underlying structure of quasicrystals, it is first necessary to have some understanding of the structure of a conventional crystal. Perhaps the most basic principal of solid state physics is that a solid is composed of atoms packed in a dense arrangement and that the ordering of the atomic arrangement determines many properties of the solid. The atomic arrangement in a solid can be compared to a mosaic. Atoms or clusters of atoms appear in repeating motifs called unit cells, which are analogous to tiles in a mosaic. The order in an arrangement of atoms (or in a mosaic) is determined by the way in which the unit cells (or tiles) are joined to form the complete structure.
Crystals have highly ordered atomic arrangements in which all the unit cells are identical, analogous to a mosaic constructed from a single tile shape, as in typical bathroom tiling. A single atomic cluster or unit-cell shape is repeated periodically (with equal spacing between cells) to form the structure. Crystals have positional order: given the position of one unit cell, the positions of all the other unit cells are determined. Crystals also have orientation order: given the orientation of one unit cell, the orientations of all the other unit cells are determined.
The orientation order can be characterized in terms of a rotational symmetry. A special set of discrete rotations leaves the orientations of the unit cells unchanged. According to the well-established, rigorous theorms of crystallography, only a small list of rotational symmetries is possible for crystals: crystals can have two-fold, three-fold, four-fold, or six-fold axes of rotational symmetry; other possibilities, such as five-fold, seven-fold, or eleven-fold symmetry are not allowed. This corresponds to the observation that one can tile a bathroom wall using a single tile shape if the tiles are all rectangles, triangles, squares, or hexagons, but not if they are pentagons. A "crystal" tiling can also be constructed if the tile shape is a parallelogram, in which case the analogous crystalline lattice exhibits no rotational symmetry.
In contrast to crystals, glass has a highly disordered atomic arrangement. A glass is typically formed by rapidly cooling a vapor or liquid well below its freezing point until the atoms are frozen into a dense but random arrangement. Window glass, the most common example, is formed by silicon and oxygen joined in a random network of covalent (directed) bonds. Physicists have succeeded in rapidly cooling various mixtures of metal atoms to form "metallic" glasses. In this case, there are no preferred directions for the bonds, and the atoms are packed in a dense but random arrangement.
A glass is analogous to a mosaic form from an indefinite number of tile shapes randomly joined together. The concept of a unit cell is normally not used in this case, since there is no well-defined scheme for dividing the atoms into infinitely many unit-cell shapes. The structure has neither positional nor orientational order: the position and orientation of one unit cell does not determine the position, orientation or shape of others a distance away.
The possibility of a new class of ordered atomic structures has been proposed by Levine, D. and P. J. Steinhardt, Quasi-Crystals: A New Class Of Ordered Solids, Phys. Rev. Lett. 53:2477, based on a detailed study of a special two-dimensional tiling pattern discovered by Penrose, R., Bull. Inst. Math. and Appl., 10:266 (1974), some ten years earlier. The new structures are analogous to mosaics with more than one tile shape but only a finite number of shapes. Although the structures have positional order, the tiles are neither periodically nor randomly spaced; instead they are quasi-periodically spaced. This means that, given the position of one tile, the positions of the other tiles are determined according to a predictable but subtle sequence which never quite repeats. The new structures also exhibit orientational order; each tile of a given shape is oriented along one of a small, discrete set of special directions. The rotational symmetry is defined by the set of rotations which leaves the set of orientations for each of indifferent tile shapes unchanged. Because the new structures are highly ordered like crystals, but are quasi-periodic instead of periodic, they have been called quasi-periodic crystals or quasi-crystals for short.
Just as the theoretical notions were being developed, Schectman, Blech, Gratias and Cahn, Metallic Phase With Long-Ranged Orientational Order And No Translational Symmetry, Phys. Rev. Lett., Vol. 53, No. 20, page 1951 (1984), were independently studying a puzzling new alloy of aluminum and manganese which, they discovered, had five-fold symmetry axes. The new material was discovered accidentally as part of an extensive survey to develop lighter and stronger aluminum alloys. It was formed by a method known as melt spinning, in which a hot liquid alloy mixture is sprayed onto a cold-spinning wheel so that the liquid rapidly solidifies. The alloys being studied by Schectman and his colleagues cooled into long strips of metal. For an appropriate mixture of aluminum and manganese, the strips contain tiny "grains," about 10 microns across, within which appeared to be a homogeneous material.
To determine the atomic structure of the new material, Schectman and his colleagues used a technique called electron diffraction analysis. They aimed a beam of electrons at a single grain of an alloy and recorded the pattern formed when the electrons scattered off the atoms in the material and struck a photographic plate. For a crystal, it is well known that the electrons scatter coherently from the positionally ordered array of atoms to form a "diffraction pattern" of sharp spots on the plate. The pattern of spots depends on the symmetry of the crystal and its orientation with respect to the electron beam. For a glass, the electrons scatter off an isotropic, disordered array of atoms to form a diffraction pattern of diffuse rings which is the same for all orientations.
For the aluminum-manganese alloy, a pattern of sharp spots was found which clearly indicated a five-fold symmetry axis. By rotating the sample in the electron beam, it was found that the material had many five-fold symmetry axes, as well as three-fold and two-fold symmetry axes. By noting the angle between the symmetry axes, it could be shown that the material had a three-dimension icosahedral symmetry, one of the most familiar examples of a disallowed crystallographic symmetry. The icosahedron is one of the five regular polyhedra that are referred to as Platonic solids. The word icosahedron means "twenty faces"; the icosahedron has twenty identical triangular faces, thirty edges, and twelve vertices. The black pentagons on the surface of a soccer ball are centered on the vertices of an icosahedron. Each of the vertices lies on one of six five-fold symmetry axes which connect opposite vertices. Because of the five-fold symmetry axes, icosahedral symmetry is disallowed for crystals. In particular, icosahedra cannot be packed so as to fill space completely, just as pentagons cannot be joined to form a complete tiling of a plane.
Although the diffraction patterns found for the new alloys are clearly impossible for crystals, they correspond very closely with the theoretical computations of the diffraction pattern expected for icosahedral quasi-crystal. Levine and Steinhardt, Quasi-Crystals: A New Class Of Ordered Solids, Phys. Rev. Lett. 53:2477 (1984); Elser, Indexing Problems In QuasiCrystal Diffraction, Phys. Rev. B., 32:4892 (1985); Kalugin, et al, Six-Dimensional Properties Of Al.sub.0.86 Mn.sub.0.14 Alloy, JETP, 41:119; J. Phys. Lett. 45:L601 (1985); Duneau and Katz, Quasi-Periodic Patterns, Phys. Rev. Lett. 54:2688 (1985), all incorporated by reference. The correspondence led to the suggestion that the new alloy may be the first example of a quasi-crystal.
The unique symmetry properties of quasi-crystals account for the distinctive diffraction pattern, the pattern produced by scattering electrons off an ideal quasi-crystalline solid. For a traditional crystallographer, one striking feature is that the pattern consists of sharp spots, just as for a periodic crystal, but with a symmetry that is disallowed for crystals. The sharp spots are the sign of positional order. The traditional crystallographer has alway associated positional order with periodicity, and therefore would expect only the usual symmetries allowed for a periodic crystal. Instead, we now see that another kind of positional order--quasi-periodic order--allows the possibility of diffraction patterns with new symmetries.
Another striking feature is that the pattern of diffraction spots is dense; in particular between any two spots there are yet more spots. In the diffraction patterns for a periodic cubic crystal, by contrast, there is an equal interval between spots along each symmetry direction, due to the fact that all the unit cells are equally separated in the atomic structure. In a quasi-crystal, however, the unit cells are separated by at least two different spacing lengths whose ratio is an irrational number. It is straightforward to show that the diffraction spots should lie at all possible integer combinations of at least two intervals whose ratio is likewise irrational. Allowing for positive and negative integer combinations a dense set of spots should appear.
Thus, when Schechtman and his colleagues reported the very unusual diffraction pattern of an aluminum-manganese alloy which consisted of a relatively dense pattern of sharp spots with an icosahedral symmetry in 1984, it was a clear signal to the theorist that the alloy might be an example of the hypothetical phase they were studying--an icosahedral quasi-crystal.